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68                               HYDROGEN  PEROXIDE

remain yellow). The vessel is then closed and shaken vigorously for half
a minute, after which 40 c.c. of N-sulphuric acid are added from, a graduated
cylinder. The vessel is again closed and left for a few minutes, the excess
of iodine being then titrated with N/io-sodium thiosulphate solution.
2 c.c. of N/io-thiosulphate = i c.c. N/5-iodine = 0-003 gram CH.X). The
number of c.c. of N/5-iodine consumed X 1-2 = grams of (TLO per 100 c.c.
This method is not applicable in presence of acetone, at any rate in appre-
ciable quantity; as a rule, however, the proportion of acetone is small.

*  *

Commercial formaldehyde, should contain not less than. 35 grams of CH,,0
per TOO c.c. Methyl alcohol is often added in the proportion of 10-25% by
volume with the view of preventing polymerisation of the aldehyde, especially
in concentrated solutions (about 40%). Occasionally the methyl alcohol is
replaced by other products, such, as ordinary alcohol, acetone;, calcium chloride,



Colourless, clear, odourless liquid. With permanganate it effervesces
briskly, liberating oxygen and decolorising the permanganate. When
acidified with a little dilute sulphuric acid, treated with a few drops of
potassium chromate and shaken with ether, the latter is coloured blue.

Commercial hydrogen peroxide almost always contains various impurities,
especially sulphuric, hydrochloric, nitric, phosphoric, silicic, hydrofluoric
and liydrofluosilicic acids (free and combined), aluminium, iron, calcium,
barium, magnesium, alkalies and ammonia. It may also contain traces
of arsenic, and, as an adulterant,'oxalic acid. The following determinations

are made.

1.  Sulphuric, Hydrochloric, Nitric and Phosphoric AcidvS, -These

are detected by the usual reagents.

2.  Silica.-—After evaporation of the sample, the residue is tested for

silica in the ordinary way.

3.  Hydro ftuosilicic Acid.- -The liquid is evaporated to small volume
and tested with potassium chloride, (white, gelatinous precipitate).

4.  Oxalic Acid.—The   liquid is treated   with   excess   of ammonia,
rendered acid with acetic acid and tested with calcium chloride.

5.  Metals, Alkaline Earths, Alkalies.......-These are detected by the

ordinary reagents   (ammonia.,  ammonium sulphide,  ammonium  oxalate,
sodium phosphate, etc.), the liquid itself or its evaporated residue being
used.    Iron may be detected with ammonium thiocyanate,

6.  Fixed Residue.—50 or 100 c.c. are. evaporated in a tared dish on
a steam-bath, the residue being dried at 110° and weighed.

7.  Acidity.—10   c.c.   arc   titrated with N/io-alkali   in   presence of


8.  Determination  of the  Active   Oxygen.™By  active  oxygen is
meant the volume of oxygen yielded by I vol. of the peroxide.    25 c.c. of
the liquid are diluted to 250 c.c. and 10 c.c. of this solution (= 1 c.c. of
the peroxide) are diluted with 100 c.c. of water, acidified with 4 c.c. oflowed to cool,